1. Field of the Invention
The invention relates generally to the control of flow phenomena, and more particularly, but not exclusively, to a robust flow control actuator capable of influencing supersonic boundary flow phenomena.
2. Description of the Related Art
Active flow control is regarded as an enabling technology for many advanced air vehicle concepts under consideration. Effective manipulation of a flow field can lead to a number of significant benefits for aerospace vehicles, including enhanced performance, maneuverability, payload, and range, as well as lowered overall cost. These macro benefits are directly achievable through the application of transition, turbulence and flow separation. Organizations such as the United States Air Force and NASA continue to investigate the potential advantages of active flow control over more traditional aerodynamic techniques.
Steering an aerodynamic body results from inducing asymmetric body forces typically produced by some sort of flow control technology. In one approach, commands from a control system vary the power into an actuator. As is well known to those skilled in the art of fluid dynamics, flows over aerodynamic surfaces typically have a high-sensitivity region, where a minimum actuator input will produce a maximum fluidic change. The flow phenomena to be controlled could be related to laminar-to-turbulent boundary layer transition, the separation of boundary layers, or acoustic disturbances. It could also be related to the control of vortices, jet vectoring, mixing or steering. An actuator for controlling the flow phenomena can be constructed based on any one of a variety of existing technologies, including, for example, fluidics, thermodynamics, acoustics, piezo-electric elements, synthetic jets, electromagnetics or Micro-Electro-Mechanical systems (MEMS).
Presently, several classes of micro-actuators are under investigation for flow control applications associated with aerospace vehicle systems. A majority of these micro-actuators use mechanical deflection of control surfaces, mass injection, or synthetic jets to manipulate boundary layer interactions. Other actuators manipulate electromagnetic fields in an attempt to control flow. Each has significant limitations for applications involving supersonic flows.
Mechanical actuators include electroactive polymers, shape memory alloys, electro-active ceramics, and MEMS. These actuators control flow by movement of a control surface to physically change the camber of an airfoil, thus changing the lift of a wing, for example. Manipulation of surface texture via MEMS tabs can induce vortices on the leading edge of a wing that affect its lift Mass injection devices include combustion-driven jet actuators, which burn a gaseous fuel-air mixture. A chamber is filled with a combustible mixture and then ignited, resulting in high pressures inside the chamber and mass expulsion through the chamber orifice. Combustion-driven jet actuators require a considerable amount of auxiliary equipment to function. A fuel source is needed. Fuel must be pumped to the point of use, metered for the proper fuel air mixture, and injected into the combustion chamber. There a precisely timed ignition source must occur to ignite the fuel. The required fuel supply, plumbing, pumps, metering devices, fuel, injectors, ignition devices, timing devices, etc. significantly complicate this approach to jet actuators. Additionally it is difficult to perform these auxiliary tasks at the macro level required for the jet actuators.
Synthetic jet actuators are fluidic control devices that transfer momentum into the external system without net mass transfer. Actuators, such as synthetic jets, that operate without net mass transfer are known as zero net mass flux (ZNMF) devices. They have been shown to be effective for low-speed (subsonic) flows, but in the past did not have the required mass flow output and high frequency for supersonic flow applications. They typically use a piezo-electric diaphragm in a cavity opposite an orifice. The oscillatory motion of the diaphragm alternately decreases the cavity volume, expelling gas, and then increases the cavity volume, refilling the cavity with gas. The oscillation frequency of piezoelectric diaphragms are governed by their size, displacement, and mass. The smaller the diaphragm the higher the frequency at which it can oscillate. Unfortunately the smaller the diaphragm the smaller the diaphragm and mass displacement and thus the smaller the jet momentum flux. Since piezo-electric diaphragms have a small displacement they need a large area to displace a useful amount of air from the cavity. This is counter productive to achieving the high frequency operation and limits the piezo-electric units to low frequency operation of a few hundred cycles per second. Small piezo-electric diaphragms can achieve frequencies in the kilohertz range, but can't produce the displacements necessary to move enough air for effective synthetic jet operation. Large power supplies are typically required to achieve the high rate of change in voltage needed to drive these piezo-electric devices. Additionally the device includes moving parts which can fatigue and fail.
Each of the afore-mentioned actuators has significant limitations for applications related to supersonic flow control in terms of the combined operating frequency and momentum flux necessary for supersonic flow applications. Accordingly, a need exists for a robust flow control actuator that is capable of influencing supersonic boundary layers.